A combination of cargo culting/"sounds professional", and just wanting to make yourself/company look less bad.

Assuming legal signed off on the basic message, I'm guessing it's out of a sense of 'politeness' in a delicate situation. Consider how someone might publicly communicate a relationship breakdown with language like "we're parting ways" or "we have made the decision to end our marriage." Sometimes you get plain, direct language but people might infer acrimony from it.

My sense is that in the particular case of layoffs, companies are hesitant to actually use the phrase "laid off" because it triggers a sense of implicit shame in those affected. There is definitely an unspoken assumption that folks who get laid off are underperformers, and the extent to which we talk about how untrue that is (see: LinkedIn) shows how pervasive the assumption is.

Cowardice

You fired someone? You're to blame. Someone was impacted? Sounds more like an act of nature. Nobody is to blame!